1. Field of the Invention
The invention is related to the field of directional surveying apparatus used in wellbores drilled through the earth. More specifically, the invention is related to apparatus and methods used to determine the orientation of a well logging instrument so that the measurements made by the instrument can be referenced with respect to a geographic benchmark.
2. Description of the Related Art
Well logging instruments include various types of sensors for measuring various properties of fluid flowing inside a wellbore. The measurements can be used for, among other things, determining the fractional volumes of different types of fluids flowing in the wellbore. The fluids can include mixtures having varying fractional volumes of gas, oil and water. Recently it has become common to drill wellbores at high angles of inclination with respect to vertical. The same types of sensors used to evaluate the fluids flowing in vertical wellbores have been adapted to determine the fractional volumes of fluids flowing in these high angle and horizontal wellbores. The adaptation of the sensors, however, typically requires that the measurements made by the adapted sensors be referenced to a benchmark such as the earth's gravity in order to be able to determine which portion of the cross-section of the wellbore is occupied by which type of fluid. Gravity is a convenient benchmark primarily because the fluids are typically segregated by gravity. It is useful, therefore, to include in such a well logging instrument a sensor which can determine the orientation of the well logging instrument with respect to gravity.
Sensors are known in the art for determining the relative orientation of a well logging instrument with respect to gravity and the earth's magnetic or geographic north pole. The sensors known in the art typically include three accelerometers positioned orthogonally to each other, and three magnetometers positioned mutually orthogonally (or alternatively, a gyroscope or similar geographic north-pole referencing device combined with a rotary orientation sensor). Combining the measurements of the accelerometers and magnetometers provides the system operator with the relative orientation of the instrument with respect to magnetic (or geographic) north and vertical (gravity).
Three orthogonal accelerometers alone could be adapted to provide a reference with respect to gravity suitable for the requirements of the fluid-flow sensor instrument previously described, but the accelerometers have several drawbacks which would make their use, without more, impracticable in such an instrument. For example, the accelerometers must be mounted in a very precisely machined fixture to assure that the sensitive axes of the accelerometers remain mutually orthogonal. The fixture with accelerometers included may require periodic calibration for orthogonality with respect to temperature since the well logging instrument may be subject to a wide range of ambient temperatures. The accelerometers themselves may require periodic calibration since the signal level, with respect to acceleration, that they generate is subject to change with time and with ambient temperature. Proper characterization of the response of the accelerometers may require a temperature sensor to be included with the instrument and the acceleration measurements to be corrected for temperature while a well log is being recorded. All of these methods of accelerometer calibration and characterization can be difficult and expensive to perform.
It is an object of the present invention to provide an inexpensive system for calibrating the orientation of a well logging instrument with respect to the earth's gravity that may be used while the instrument is disposed in the wellbore and does not require expensive and difficult periodic calibration to a benchmark standard.